People reports that the former couple were there to celebrate the launch of Pink + Dolphin's flagship store, but Chris spent the evening hanging out with Christina Milian and Sean Kingston, which upset Rihanna.

The songbird, who recently told Oprah she still loved Chris, tried to act like she didn't care, but a source says she did.

"[She was] pretending to dance and mingle," but kept her eyes on "all the fun Chris and Christina were having."

Rob Kardashian was seen chatting it up with the 24-year-old for a few, but, "You could tell she really just wanted Chris to pay attention and talk to her, but he was totally ignoring her," the witness added.